Add Windows CI

[MSP-Greg]

Adds a GitHub Actions workflow that only runs bundle exec rake compile. It has code to always pass, so it won't 'fail' a PR.

Several errors/warnings are shown, some were fixed in #966.

Notes:

1. This is only using a Ruby built with the MSYS2 UCRT64 gcc tool chain. I believe additional code is needed to even try to build with a mswin (MSVC) Ruby.
2. First time this runs, installing/compiling Nokogiri takes several minutes. Subsequent builds can make use of the cache, so they're much quicker.
3. Once it compiles, addtional code is needed to load librubyparser.dll. Something similar to the below:

require 'fiddle'
kernel32 = Fiddle.dlopen 'kernel32'
load_library = Fiddle::Function.new(
  kernel32['LoadLibraryW'], [Fiddle::TYPE_VOIDP], Fiddle::TYPE_INT,
)
dll_path = File.absolute_path('../build/librubyparser.dll', __dir__)
if load_library.call(dll_path.encode('utf-16le')).zero?
  abort "Failed to load librubyparser.dll from\n  #{dll_path}"
end

[flavorjones]

FWIW I also have a branch in progress with windows support that is also going to introduce static linking of the ruby C extension.

[kddnewton]

Thank you so much for the help @MSP-Greg! I see that Mike credited you on some of the commits he added recently. I'm going to close this one in favor of that other one.